I’m afraid I have nothing to contribute, other than what you probably have found already in the St. Senan parish baptism register (1827-1863) and marriage register (1829-1881). There is only one child of John Nash and Jane Quilty in the baptism register, and she was aged 20 when she was baptised:
23.07.1867: Baptism of Sarah Nash of John Nash and J. Quilty; sponsors: Patrick O’Kelly, Mrs. Ivers; Priest’s note: a convert, aged 20.
Sarah married the following day:
23.07.1867: Marriage of Sarah Nash, Moore Street, to Joseph Garry*, Henry Street, Kilrush; witnesses: Michael Egan, Mrs. Thomas Ivers; Priest’s note: a convert baptised previous day.
*The person who reported the death of John Nash in 1880 was this Joseph Garry.
It looks like Sarah Nash converted to Roman Catholicism in order to marry Joseph Garry, and that her parents, John and Jane, were of another religion. This may explain why they do not appear as parents in the register of baptisms, but there may be another explanation. Sarah was aged 20 in 1867, so born about 1847. Her marriage to Joseph Garry was registered in Kilrush, but the image of the record is not available on line: http://www.irishgenealogy.ie
Given that John was born about 1815, I imagine that he married about 1835-1840, and that Sarah’s siblings were born in the 1840s. I think that if any of Sarah’s siblings remained in Kilrush they also converted to Roman Catholicism – at least the Nashes living in Kilrush in 1901 are all Roman Catholics.
I see several baptisms of children of Joseph Garry and Sarah Nash, but there are no Nashes among the sponsors – maybe Sarah’s family did not approve of her conversion, or, again, maybe there is another explanation.
Joseph and Sarah are in the 1901 census, living in Ballymaccurtain (Kilrush Urban); Joseph’s occupation: Civil Bill Officer. One of their sons is a Farmer, and two others are Harness makers. One of their daughters is a Monitress. There are also two apprentice Harness makers living in the house. They seem to be comfortably off.
The 1911 census shows Sarah only, a widow, aged 70. In this census Sarah gives her birthplace as Dublin – whereas in 1901 she gives it as Clare. Was Sarah really was born in Dublin? If so, her father may have been from Dublin, but, if so, he moved to Kilrush and remained there until his death in 1880. And did Sarah's siblings remain in Dublin, or did they also move? Richard, I assume that you are a descendant of a brother of Sarah. Have you found any record of that brother’s birth, or of his marriage?